Refactor WebKitClient into protocol, target, command, script, and event modules

[shaun0927]

Tracked by #713.

Background

src/webkit/client.ts is a central 1,316-line module that mixes protocol transport, target multiplexing, browser commands, DOM script templates, event adapters, and errors. This is maintainability risk for a core automation path.

Objective

Split WebKitClient internals into focused modules while preserving the public BrowserBackend-facing facade.

Scope

• Extract protocol transport without changing public methods.
• Extract DOM script builders used by browser commands.
• Extract typed event adapters.
• Keep WebKitClient as compatibility facade during migration.

Target files and paths

• src/webkit/client.ts
• new src/webkit/protocol-transport.ts
• new src/webkit/target-session.ts
• new src/webkit/browser-commands.ts
• new src/webkit/dom-scripts.ts
• new src/webkit/events.ts
• new src/webkit/errors.ts
• tests/unit/webkit*.test.ts

Implementation steps

1. Create error module and update imports.
2. Extract pure DOM script builders with tests.
3. Extract protocol send/pending-request handling behind an interface.
4. Move target lifecycle state into a target session module.
5. Keep facade methods and run existing tests after each extraction.

Acceptance criteria

• Public WebKitClient constructor and BrowserBackend methods remain compatible.
• Extracted modules have focused tests or existing tests covering their behavior.
• No CDP or Playwright abstractions are introduced.
• File size of client.ts is materially reduced without a behavior rewrite.

Verification

• npm run lint -- --quiet
• Relevant WebKit unit tests
• npm run build:cli
• Manual smoke: navigate, screenshot, javascript, inspect against Safari simulator.

Dependencies / recommended order

Run after WebKit hot-path tests exist. Split into multiple PRs; do not attempt the full extraction in one PR.

Risks and cautions

• This is a refactor; behavior changes need separate issues.
• Avoid broad renames that make review difficult.
• Keep WebKit protocol constraints from CLAUDE.md.

Non-goals

• Do not change tool names or schemas.
• Do not migrate to Playwright/CDP.
• Do not combine with HTTP security work.

Ambiguity reducers

• “Facade” means external imports can still use WebKitClient from the same path until a deprecation plan exists.
• Each extraction PR must have a small, reviewable diff.
• Pure script builders should be testable without simulator.

Validity review

Proceed after higher-priority safety and speed issues. The work is valid because WebKitClient is a core change surface.

Implementation detail refinements

• Use adapter interfaces to avoid circular dependencies.
• Move tests with the code where practical.
• Track line-count reduction as a secondary metric, not the success criterion.

Verification checklist reinforcement

• Existing WebKit tests pass after each extraction.
• At least one pure module has direct unit coverage.
• Manual Safari smoke passes at the end of the final PR.
• No public API break is introduced.